Fusobacterium species are anaerobic Gram-negative microorganisms that are commonly found in the mouth. Fusobacterium nucleatum is the most frequently isolated pathogen from periodontal disease sites and is believed to be an initiator of periodontal diseases (see, e.g., Moore & Moore, Periodontology 2000 5:66-77 (1994)). Moreover, this bacterium is commonly found in abscesses and other infections in the abdomen, blood, chest, lung, sinuses, and female genital tract (see, e.g. Brook, J. Clin. Microbiol. 26:1181-1188 (1988); Hoist et al., J. Clin. Microbiol. 32:176-186 (1994); Moore & Moore, supra). F. nucleatum is usually found as a component of polymicrobial infections, but is also found as a single isolate from infections, demonstrating its pathogenic potential. The virulence properties of F. nucleatum are related to its adherence to host tissues and other bacterial species, as well as its modulation of host cell immune function (see, Bolstad, Clin. Microb. 9:55-71 (1996)) Thus, F. nucleatum is a microorganism of interest for further investigation because of its role in the periodontal diseases and other human infectious diseases.
Thus far, several F. nucleatum proteins that are believed to be associated with F. nucleatum pathogenesis have been cloned and expressed in Eschericia coli. For example, a fomA gene that encodes the major outer membrane protein has been cloned. This protein functions as a porin and may act as a receptor in coaggregation with other pathogenic bacteria (see, Jensen et al., Microb. Path. 21:331-342 (1996); Kinder Haake et al., Arch. Oral Biol. 42:19-24 (1997)). Also cloned is a fipA gene which encodes an immunosuppressive factor that inhibits human T-cell responses (see, Demuth et al., Infect. Immun. 64:1335-1341 (1996)). Moreover, a homologous family of small cryptic plasmids in strains of F. nucleatum has been isolated (McKay et al., Plasmid 33:15-20 (1995)). However, the molecular manipulation of genes in F. nucleatum has not been possible due to the lack of reliable gene transfer and expression systems for F. nucleatum. The development of gene transfer and expression systems for F. nucleatum is essential to express and fully characterize the cloned F. nucleatum genes in the native host background.
Therefore, there is a need for reliable gene transfer and expression systems for F. nucleatum. The availability of gene transfer and expression systems would assist the molecular characterization of genes that are associated with pathogenesis of F. nucleatum. The availability of such systems would further assist the development of compounds that can prevent or treat periodontal diseases or other human infectious diseases caused by F. nucleatum. The present invention fulfills this and other needs.